In an Organic Reaction Mechanism, a Reaction Intermediate is a molecular entity that is formed by the reactants which reacts further to form the products. Although, there are a multitude of organic reactions with multiple mechanisms and intermediates, there are a few reaction mechanisms which form the foundations of organic reactions. One of these is the Nucleophilic Substitution Reaction which involves an electron nucleophile attacking a positively charged atom (which is usually a carbon) in order to substitute a leaving group. These reactions can be broken down into two further categories: SN1 and SN2.
SN1 reactions are substitution reactions where the rate-determining step is unimolecular. This suggests that the mechanism is only dependent on the electrophile, and totally independent of the nucleophile. Knowing these details, it can be inferred that the intermediate that forms in the reaction, if the electrophile is carbon-based, is a carbocation. After the carbocation forms, then the nucleophile will subsequently attack it based on the opposition of charge. Thus the leaving group was replaced by a nucleophile – hence the term SN1 (unimolecular nucleophilic substitution reaction).
SN2 reactions are also substitution reaction, but with a different reaction mechanism, as it proceeds in a bimolecular manner. The nucleophile in this case attacks the electrophile without a leaving group leaving. Thus, a temporary transition state with five substituents around the carbon is formed. Eventually, the leaving group leaves and the structure around the central carbon inverses.
Although nucleophilic substitution reactions are prevalent in Organic Chemistry, there are other important reactions such as Elimination Reactions, which may share some of the reaction mechanisms and intermediates, such as the carbocations. Thus, in Organic Chemistry, it is most often the physical and chemical conditions which determine which intermediates form as well as the direction of a particular reaction.